Tetradentate Bis(o-iminobenzosemiquinonate(1-)) π Radical Ligands and Their o-Aminophenolate(1-) Derivatives in Complexes of Nickel(II), Palladium(II), and Copper(II)

The coordination chemistries of the potential tetradentate ligands N,N′-bis(3,5-di-tert-butyl-2-hydroxyphenyl)ethylenediamine, H ₄[L¹], the unsaturated analogue glyoxal-bis(2-hydroxy-3,5-di-tert-butylanil), H₂[L²], and N,N′-bis-(2-hydroxy-3,5-di-tert-butylphenyl)-2,2-dimethylpropylenediamine, H₄[L³], have been investigated with nickel(II), palladium(II), and copper(II). The complexes prepared and characterized are [Ni^II(H₃L¹)₂] (1), [Ni ^II(HL²)₂]·5/8CH₂Cl ₂ (2), [Ni^II(L^3··)] (3), [pd ^II(L^3··)][pd^II(H ₂L³) (4), and [Cu^II(H₂O)(L ⁴)] (5), where (L⁴)^2- is the oxidized diimine form of (L³)^4- and (L^3··) ^2- is the bis(o-iminosemiquinonate) diradical form of (L ³)^4-. The structures of compounds 1-5 have been determined by single crystal X-ray crystallography. In complexes 1 and 2, the ligands (H₃L¹)^- and (HL²) ^- are tridentate and the nickel ions are in an octahedral ligand environment. The oxidation level of the ligands is that of an aromatic o-aminophenol. 1 and 2 are paramagnetic (μ_eff ∼ 3.2 μ _B at 300 K), indicating an S = 1 ground state. The diamagnetic, square planar, four-coordinate complexes 3 and [pd^II(L ^3··)] in 4 each contain two antiferromagnetically coupled o-iminobenzosemiquinonate(1-) π radicals. Diamagnetic [pd ^II(H₂L³)] in 4 forms an eclipsed dimer via four N-H⋯O hydrogen bonding contacts which yields a nonbonding Pd⋯Pd contact of 3.0846(4) A. Complex 5 contains a five-coordinate Cu^II ion and two o-aminophenolate(1-) halves in (L⁴) ^2-. The electrochemistries of complexes 3 and 4a ([Pd ^II(L^3··)] of 4) have been investigated, and the EPR spectra of the monocations and -anions are reported.